Reconstructed Viceroy Butterfly
I’m afraid this one isn’t very “Christmassy”. Sorry about that.
On June 15, Sandy’s parents came up to visit us (and particularly to see their granddaughters). This was a long trip, it’s roughly 570 miles from their house to here. And somewhere along the way, their radiator grille intercepted this butterfly that Sandy and Sam found still stuck there. The body was . . . well, never mind the body, but I was able to reconstruct the wings pretty much.
At first glance, it appeared to be an unfortunate Monarch butterfly. The orange and black pattern on the forewings in particular is spot on:
But, there are two things that tell us it wasn’t a Monarch: first, it was noticeably smaller, and second, there is an extra band of black across the hindwing:
This tells us that what we actually have here is the remains of a Viceroy butterfly, Limenitis archippus, which is an excellent Monarch mimic. It’s convenient for us that the distinguishing black line on the hindwing still existed, otherwise we never would have been quite sure what this one was.
Viceroys are frequently held up as a classic example of Batesian mimicry, where a harmless creature is disguised as something poisonous, dangerous, or inedible so that predators will avoid it. In this case, Monarch butterflies are notably foul-tasting and toxic because of the chemicals they pick up from milkweed, and so most birds will avoid eating anything that resembles a Monarch.
However. . .
It has recently been determined that Viceroy butterflies are also foul-tasting, and perhaps even more foul to certain predators than Monarchs are. Which makes this a case of Mullerian mimicry, where two different poisonous, dangerous, or inedible species grow to look like each other so that each species can get the benefit of predators learning not to eat the other. See, if you have two different inedible species that look different, then a bird will try eating one, learn not to eat that one, but then still have to eat one of the other to learn not to eat them. But, if the two species look the same, then a predator just has to eat one of them in order to learn not to eat both of them.
While the adult Viceroy and adult Monarch look a lot alike, their caterpillars are radically different. The Viceroy caterpillar is basically disguised as a bird dropping, and eats mostly willow and poplar. Both of these trees have a certain amount of salicylic acid in them (a chemical closely related to aspirin), and the caterpillars reportedly sequester the salicylic acid in their bodies as their defensive chemical.
There are a couple of generations of Viceroy butterfly per year, with the caterpillars overwintering by wrapping themselves in a dead leaf on the ground. Then in the spring they come out, eat some fresh leaves for two to four weeks, and then pupate. And, when they emerge as adults, they are just in time to blend in with the Monarchs coming back up from the south.
And get hit by a car. Ah well, these things happen, I guess.
 Yes, convenient, that black band . . . almost too convenient. Why would the mimicry be so good overall, and yet leave such a clear distinguishing feature? I have to say I don’t really know. It implies that there is some reason why the Viceroys sometimes need to be distinguishable from Monarchs, but only by those “in the know”. I wonder if Viceroys find mates by sight, and so need a visual marker so that they can tell each other from Monarchs?
 See, this is what happens when one just goes around making assumptions about the palatability of insects, rather than going ahead and tasting them. If somebody had just tasted a Viceroy years ago, they never would have made this mistake.
 So, presumably eating a Viceroy would be similar to eating an aspirin. Which is bitter. And could cause stomach bleeding. Although Viceroy butterflies might be good for headaches. And maybe even prevent heart disease. Or cancer.
 Yes, yes, I know; the salicylic acid in willow trees and Viceroys isn’t the same as the acetylsalicylic acid in aspirin. And neither of them is the same as the methyl salicylate in oil of wintergreen, or the bismuth subsalicylate in Pepto-Bismol. But they are all generally related compounds with similar (although obviously not identical) anti-inflammatory properties. It looks like salicylic acid from willow bark was the original drug, and acetylsalicylic acid was developed as a substance that was less irritating to the stomach but was still effective for pain relief.